In recent years, in hygiene materials such as disposable diapers and sanitary napkins, what is called incontinent pads, the water-absorbing resin as a composition material thereof has been widely used, aiming at absorbing body liquid. As such a water-absorbing resin, for example, there have been known a cross-linked product of partially neutralized polyacrylic acid, a hydrolysate of a starch-acrylic acid graft polymer, a saponified product of a vinyl acetate-acrylate ester copolymer, a hydrolysate of an acrylonitrile copolymer or an acrylamide copolymer or a cross-linked product thereof, and a cross-linked polymer of a cationic monomer and the like.
As property required for these water-absorbing resin powders, accompanying with high functionalization of disposable diapers, which are principal applications, many properties have been required, such as, not only absorbency against non-pressure (CRC) and soluble components (“Extractables”) as conventional fundamental properties, but also absorbency against pressure (AAP), water-absorbing speed (FSR/Vortex), liquid permeation, suction force, coloring prevention characteristics (whiteness after production and in time course), residual monomers, urine resistance, powder substance fluidity and the like, as well as level thereof has ever become higher.
Accompanying with multi-functionalization and tendency to higher performance of the water-absorbing resin, there has been tendency that many steps are added to a conventional production step of the water-absorbing resin, such as the surface cross-linking step or an addition step of various kinds of additives. Therefore, due to incorporating a plurality of steps complicatedly, it has become increasingly difficult to produce the water-absorbing resin with multi-function and high performance in a large industrial scale.
Accordingly, as compared with a conventional production process of the water-absorbing resin, decrease or deflection (variation width relative to desired property value) of property, or a product out of specification (product out of specification in property) is generated, accompanying with scale up, and there was also a case of generating a problem of not only property decrease but also cost up or environmental load with disposal. This tendency has become more remarkable with expansion of production scale of the water-absorbing resin per one apparatus, in ever expanding demand of the water-absorbing resin in recent years.
Therefore, there have been proposed the following Patent Documents 1 to 11, in producing the water-absorbing resin with high property, and in high productivity.
In order to attain high property and high production amount, the following methods have been proposed: A method for controlling neutralization of acrylic acid, which is a raw material of the water-absorbing resin (Patent Documents 1, 2), a method relating to a specified amount of inhibitor in the polymerization step and operation control of a polymerization apparatus (Patent Document 3), a method for adding a water-soluble inorganic base in surface cross-linking the water-absorbing resin (Patent Document 4), a method for using two or more hoppers in a production line of the water-absorbing resin (Patent Document 5), a method for controlling residual monomers by each particle size of the water-absorbing resin by adding an aqueous solution of a sulfur-containing reducing agent (Patent Document 6), a method for polymerization in two polymerization systems (polymerization lines) of the same method and producing in the last half of the production step (the drying step or the like) in one line (Patent Document 7), a method for removing the water-absorbing resin with equal to or higher or lower than predetermined property, and returning it to the production step for mixing (Patent Document 8) and the like.
In addition, in order to suppress aggregation in the production step, there have been proposed a method for heating or retaining heat of an apparatus in a step of storage, pulverization, transportation or the like (Patent Document 9), a method for performing polymerization and drying at the same time by heat of polymerization (Patent Document 10) and the like.
Other than these, a method for using pneumatic transportation for uniform mixing of inorganic powders such as silica fine particles into the water-absorbing resin (Patent Document 11) has been known as well.
However, even by these methods, it was not able to suppress decrease or deflection of property and generation of a product out of specifications sufficiently, accompanying with production scale up of the water-absorbing resin (in particular, production of equal to or higher than 100 Kg/hr), and in addition there was the case of significant decrease in productivity accompanying with complication of the steps. In particular, in a conventional production method, in producing in a production scale of equal to or higher than 100 kg, still more equal to or higher than 500 kg, and in particular, equal to or higher than 1 ton per one hour, variation width (deflection width) of property by each lot became large, and for example, in sampling the water-absorbing resin by each 1 hour, or in sampling by each production amount of 100 kg or 1 ton in the production line for property evaluation, there was the case where a product out of specifications was produced.
PRIOR DOCUMENTS
Patent Documents
Patent Document 1: WO 2007/28747 pamphlet
Patent Document 2: WO 2007/28751 pamphlet
Patent Document 3: WO 2006/34806 pamphlet
Patent Document 4: WO 2002/100451 pamphlet
Patent Document 5: U.S. Pat. No. 6,727,345
Patent Document 6: EP-A-1426157
Patent Document 7: WO 2007/023097 pamphlet
Patent Document 8: U.S. Pat. No. 7,193,006
Patent Document 9: U.S. Pat. No. 6,817,557
Patent Document 10: U.S. Pat. No. 4,985,518
Patent Document 11: US-A-2007-0225160